Optical radar for ranging became feasible with the advent of the laser. Target range (distance) measurement has been effected with laser diodes by modulating the current used to drive the laser diode, for example as disclosed in an article "Range Finding Using Frequency-modulated Lase Diode" by G. Behiem and K. Fritsh, Applied Optics 25, 1439 (1986). This modulation introduces a so-called chirp frequency into the laser emission. Backscatter from a target, combined with the frequency chirp, introduces an amplitude modulation of the laser output which is used for computing range.
Further results of self-coupling of a laser diode to an external reflector are presented in an article "Laser Diode Feedback Interferometer for Stabilization and Displacement Measurements", by T. Yoshino, M. Nara, S. Mnatzakanian, B. S. Lee, and T. C. Strand, Applied Optics 26, 892 (1987). This article is directed toward measurements related to alterations to the optical path between the diode and the reflector.
Most investigations and utilizations of laser diodes for ranging and velocimetry, including those of the foregoing references, have been with single-mode (i.e. single frequency) laser operation, or at least where modes are not important. An article "Small Laser Dopper Velocimeter Based On The Self-mixing Effect In A Diode Laser", by H. W. Jentink, F. F. M. de Mul, H. E. Suichies, J. G. Aamoudse, and J. Greve, Applied Optics 27, 379 (1988) teaches multimode operation. This article is directed primarily to velocimetry but mentions some results on a cyclic dependence of Doppler strength on range, based on measurements of total laser output power.